Magnetism

Researchers at Shenzhen University, the Japan Atomic Energy Agency, the National Institutes for Quantum Science and Technology (QST), the University of Tokyo, the University of Science and Technology of China, Xi'an Jiaotong University, Shenzhen Technology University (SZTU), and The Hong Kong Polytechnic University, have grown a wafer-scale hyper-porous graphene network (HGN) that shows intrinsic ferromagnetism at room temperature. 

Optical image of HGN on a 2-inch Si wafer. Image from: Nanowerk

The film was produced across a full 2-inch oxide-coated silicon wafer using a plasma-assisted vapor process combined with low-energy electron irradiation, and reached a magnetic moment density roughly 1,000 times higher than defected highly oriented pyrolytic graphite (HOPG).

The generation of electricity from heat, also known as thermoelectric energy conversion, is advantageous for various real-world applications. For instance, it proved useful for the generation of energy during space expeditions and military missions in difficult environments, as well as for the recovery of waste heat produced from industrial plants, power stations or even vehicles.

The successful conversion of heat into electricity relies on one of two distinct effects, known as the Seebeck effect and the Nernst effect. The Seebeck effect occurs when two dissimilar materials are joined at two junctions that are at different temperatures, which can generate an electric current flowing in the loop. The Nernst effect, on the other hand, entails the generation of a transverse voltage in a material with a temperature gradient.

Researchers from MIT, University of Basel, Florida State University and National Institute for Materials Science in Japan have reported a "chiral superconductor" - a rhombohedral tetra- and penta-layer graphene material that conducts electricity without resistance, and is also, paradoxically, intrinsically magnetic. The team has found that when four or five sheets of graphene are stacked in this "rhombohedral" configuration, the resulting structure can exhibit exceptional electronic properties that are not seen in graphite as a whole.

In their new study, the scientists isolated microscopic flakes of rhombohedral graphene from graphite, and subjected the flakes to a battery of electrical tests. They found that when the flakes are cooled to 300 millikelvins (about -273 degrees Celsius), the material turns into a superconductor, meaning that any electrical current passing through the material can flow through without resistance. They also found that when they swept an external magnetic field up and down, the flakes could be switched between two different superconducting states, just like a magnet. This suggests that the superconductor has some internal, intrinsic magnetism. Such switching behavior is absent in other superconductors.

Researchers from MIT, Princeton University, SLAC National Accelerator Laboratory and Japan's National Institute for Materials Science have created a new ultrathin 2D material with unusual magnetic properties that initially surprised the researchers before they went on to solve the complicated puzzle behind those properties’ emergence. As a result, the work introduces a new platform for studying how materials behave at the most fundamental level — the world of quantum physics.

The scientists, led by MIT's Pablo Jarillo-Herrero, worked with three layers of graphene. Each layer was twisted on top of the next at the same angle, creating a helical structure reminiscent of a DNA helix.

Researchers from the National University of Singapore (NUS), working with teams from University of California, Kyoto University and others, have reported a breakthrough in the development of next-generation graphene-based quantum materials, opening new horizons for advancements in quantum electronics.

An atomic model of the Janus graphene nanoribbons (left) and its atomic force microscopic image (right). Image credit: NUS
 

The innovation involves a novel type of graphene nanoribbon (GNR) named Janus GNR (JGNR). The material has a unique zigzag edge, with a special ferromagnetic edge state located on one of the edges. This unique design enables the realization of one-dimensional ferromagnetic spin chain, which could have important applications in quantum electronics and quantum computing.

Researchers from Helmholtz-Zentrum Dresden-Rossendorf, Universität Duisburg-Essen, CENTERA Laboratories, Indian Institute of Technology, University of Maryland and the U.S. Naval Research Laboratory have used graphene disks to demonstrate light-induced transient magnetic fields from a plasmonic circular current with extremely high efficiency. 

The effective magnetic field at the plasmon resonance frequency of the graphene disks (3.5 THz) is evidenced by a strong ( ~ 1°) ultrafast Faraday rotation ( ~ 20 ps). In accordance with reference measurements and simulations, the team estimated the strength of the induced magnetic field to be on the order of 0.7 T under a moderate pump fluence of about 440 nJ cm⁻².

Researchers at the National University of Singapore (NUS), University of Science and Technology of China and the National Institute for Materials Science in Japan have developed a way to induce and directly quantify spin splitting in two-dimensional materials.

Using this concept, they have experimentally achieved large tunability and a high degree of spin-polarization in graphene. This research achievement can potentially advance the field of two-dimensional (2D) spintronics, with applications for low-power electronics.

A team of scientists, led by David Serrate, CSIC scientist at the Instituto de Nanociencia y Materiales de Aragón, INMA (a joint institute of the CSIC and the University of Zaragoza), has imaged for the first time the magnetic behavior of a graphene nanostructure. The team has not only revealed the magnetic state of narrow graphene ribbons (~2 nm), but has also shown the method they developed to magnetically characterize any planar nanographene.

Starting with a specifically designed organic precursor, the researchers synthesized the ribbons directly onto a magnetic surface, obtaining atomically precise edges that contain an alternating sequence of zig-zag graphene segments. This geometry strongly confines the graphene electron cloud around the edge, which causes the instability responsible for the intrinsic magnetism of the graphene nanostructure –a remarkable fact taking into account that the ribbon is formed just by non-magnetic carbon and hydrogen atoms.

Researchers at MIT, Harvard and Japan's National Institute for Materials Science have reported a surprising property in graphene: When stacked in five layers, in a rhombohedral pattern, graphene displays a rare, “multiferroic” state, in which the material exhibits both unconventional magnetism and an exotic type of electronic behavior, which the team has named "ferro-valleytricity".

“Graphene is a fascinating material,” said Long Ju, assistant professor of physics at MIT. “Every layer you add gives you essentially a new material. And now this is the first time we see ferro-valleytricity, and unconventional magnetism, in five layers of graphene. But we don’t see this property in one, two, three, or four layers”. The discovery could promote ultra-low-power, high-capacity data storage devices for classical and quantum computers.

Researchers at the University of Texas at El Paso (UTEP) have developed a highly magnetic quantum computing material — 100 times more magnetic than pure iron — that functions at room temperature. The team introduced an aminoferrocene-based graphene system with room temperature superparamagnetic behavior in the long-range magnetic order.

Quantum computing has the potential to revolutionize the world, allowing massive health and science computation problems to be solved exponentially faster than by classic computing. However, before this could happen, it is vital to overcome the current drawback of only operating in subzero temperatures. “In order to make quantum computers work, we cannot use them at room temperature,” said Ahmed El-Gendy, Ph.D., an associate professor of physics at The University of Texas at El Paso. “That means we will need to cool the computers and cool all the materials, which is very expensive.”